Objective: Using the unique advantages of Item Response Theory (IRT), we developed unidimensional and multidimensional models to investigate non-tremor and tremor MDS-UPDRS Part 3 changes occurring longitudinally in an interventional PD study.

Background: Although non-tremor and tremor Part 3 MDS-UPDRS items measure different impairment domains, their distinct progression and drug responsivity patterns have not been studied longitudinally.

Method: With unidimensional longitudinal IRT, we conducted a post hoc analysis of the 33 Part 3 item data of 336 early PD subjects from the STEADY-PD-III (placebo vs. isradipine) study. With multidimensional longitudinal IRT, we separated the 23 non-tremor items from the 10 tremor items. The rates of progression, related to time and treatment, in overall motor severity and in non-tremor and tremor domains were assessed using Markov Chain Monte Carlo implemented in Stan.

Results: Regardless of treatment, subjects showed statistically significant but different time-based deterioration rates for total motor (0.123 theta values per year), non-tremor (0.120 theta values per year), and tremor scores (0.123 theta values per year). Isradipine was associated with additional significant deterioration over the placebo group both in Part 3 total score (added rate of 0.065 theta values per year) and non-tremor sum score (added rate of 0.075 theta values per year), but not in the tremor sum score. Further highlighting the different progression patterns, non-tremor and tremor severity changes were positively but weakly correlated (correlation coefficient 0.108).

Conclusion: Longitudinal IRT analysis of MDS-UPDRS is a novel statistical method applicable to PD clinical trials. It addresses the limitations of traditional linear regression approaches and is particularly useful because it can separately monitor non-tremor and tremor changes, both over time and in response to treatment interventions.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/novel-approach-to-mds-updrs-monitoring-in-clinical-trials-longitudinal-item-response-theory-models/